Electrophoretic displays are known since long, for example from U.S. Pat. No. 3,612,758. The fundamental principle of electrophoretic displays is that the appearance of an electrophoretic media encapsulated in the display is controllable by means of electrical fields. To this end the electrophoretic media typically comprises electrically charged particles having a first optical appearance (e.g. black) contained in a fluid such as liquid or air having a second optical appearance (e.g. white) different from the first optical appearance. Alternatively the media might be transparent and comprise two different type of particles having different colors and opposite charge.
The display typically comprises a plurality of pixels, each pixel being separately controllable by means of electric fields supplied by electrode arrangements. The particles are thus movable by means of an electric field between visible positions, invisible positions, and possibly also intermediate semi-visible positions. Thereby the appearance of the display is controllable. The invisible positions of the particles can for example be in the depth of the liquid or behind a black mask.
A more recent design of an electrophoretic display is described by E Ink Corporation in, for example, WO99/53373. Electrophoretic medias are known per se from e.g. U.S. Pat. Nos. 5,961,804, 6,120,839, and 6,130,774, and can be obtained from, for example, E Ink Corporation.
Grayscales or intermediate optical states in electrophoretic displays are generally provided by applying voltage pulses to the electrophoretic media for specified time periods, such that the particles are moved to intermediate, semi-visible positions. The implementation of grayscales in electrophoretic displays is however connected with a number of problems. A fundamental problem is that it is very difficult to accurately control and keep track of the actual positions of the particles in the electrophoretic media, and even minor spatial deviations might result in visible grayscale disturbances.
Typically, only the extreme states are well defined (i.e. the states where all particles are attracted to one particular electrode). In case a potential is applied forcing the particles towards one of the extreme states, all the particles will be collected essentially in that particular state if the potential is applied long enough. However, in intermediate states (gray levels) there will always be a spatial spread among the particles, and their actual positions will depend upon a number of circumstances, which can be controlled only to a certain degree. Consecutive addressing of intermediate gray levels is particularly troublesome. In practice, the actual grayscale is strongly influenced by image history (i.e. the preceding image transitions), the waiting time (i.e. the time between consecutive addressing signals), ambient temperature and humidity, lateral non-homogeneity of the electrophoretic media etc.
Furthermore, accurate addressing of an electrophoretic media is obstructed by an inertia experienced in the particles. As it turns out, the particles do not respond immediately to an electrical field but instead requires a certain activation time when addressed, which results in increased image retention. To this end, the non-pre-published patent applications in accordance to applicants docket referred to as PHNL020441 and PHNL030091, which have been filed as European patent applications 02077017.8 and, 03100133.2, suggest to minimize the image retention by using preset pulses (also referred to as shaking pulses). Preferably, the shaking pulse comprises a series of AC-pulses. However, the shaking pulse may alternatively comprise a single preset pulse only.
Each shaking pulse (i.e. each preset pulse) has an energy that is sufficient to release particles present in one of the extreme positions, but insufficient to move the particles substantially. The shaking pulses thereby increase the mobility of the particles such that the subsequent drive or reset pulse has an immediate effect.
According to the co-pending European application 02079203.2 (=PHNL021000), the gray level accuracy can be further improved using a rail-stabilized approach, which means that the gray levels are always addressed via a well defined reset state, typically one of the extreme states (i.e. one of the rails). The benefit of this approach is that the extreme states are stable and well defined, as opposed to the less well defined intermediate states. The extreme states are thus used as reference states for each grayscale transition.
Theoretically the uncertainties in each gray level therefore depend only upon the actual addressing of that particular gray level, since the initial position is well known.
However, when using this approach grayscale transitions become visible as flicker, since a transition from one gray level to another includes an intermediate transition where the pixel is in one of the extreme states. This flickering effect can be reduced in case the reset state is chosen to be the particular extreme state that is closest to the previous and/or subsequent states.
For example, in a black and white display the reference initial rail state for a grayscale transition is chosen according to the desired gray level. The gray levels between white (100% bright) and middle gray (50% bright) are achieved starting from the white reference state, and gray levels between full dark (0% bright) and middle gray (50% bright) are achieved starting from the black reference state. The advantage of this method is that an accurate grayscale can be addressed with a minimum of flickering and a reduced image update time.
According to the above principle each grayscale transition thus includes a reset pulse, which resets the pixel in the respective extreme state, and an addressing pulse, which sets the pixel in the desired grayscale state. Theoretically, the duration of a reset pulse need not be longer than the time required for the particles to travel from the present state to the selected extreme state. However, using such a limited reset pulse does not actually reset the pixel completely. In fact, the appearance of the pixel still depends upon the addressing history of the pixel to some degree.
Therefore, the co-pending European application EP 03100133.2 (PHNL030091) proposes a further improvement by the use of an over-reset voltage pulse, extending the duration of the reset pulse. The reset pulse thereby consists of two portions: a “standard reset” portion and an “over-reset” portion. The “standard reset” requires a time period that is proportional to the distance between the present optical state and the extreme state. The “over-reset” is needed for erasing pixel image history and improving the image quality.
Using the reset pulse, the pixels are first brought to a well-defined extreme state before the drive pulse changes the optical state of the pixel in accordance with the image to be displayed. This improves the accuracy of the gray levels. The “over-reset” pulse and the “standard reset” pulse together have an energy which is larger than required to bring the pixel into the extreme state. Unless explicitly mentioned, for the sake of simplicity, the term reset pulse in the following refers to reset pulses without an “over-reset” pulse as well as to reset pulses including the “over-reset” pulse.
However, when the “over-reset” approach is employed the total reset period is always longer than the actual grayscale driving pulse (i.e. the pulse that moves the particles from the selected extreme state to the desired gray level), leading to the build-up of a net remnant DC voltage in the pixel. The remnant DC is actually built up and stored to some extent in the display media. The remnant DC therefore has to be timely removed or at least reduced in order to avoid gray scale drift in the subsequent image updates. In case the reset state continuously shifts between the two extreme states, the drift problem is substantially eliminated since the integral remnant DC voltage is thereby kept close to zero. However, in practice, the image sequences are often not random, and dark gray to dark gray or light gray to light gray transitions may repeatedly occur. The remnant DC is then integrated with an increased number of consecutive image transitions via the same extreme state, leading to a large grayscale drift towards that particular extreme state in subsequent image transitions. The probability of having these repetitions is particularly high if the display has a large number of gray levels.
The complete voltage waveform that has to be presented to a pixel during an image update period is referred to as the drive voltage waveform or simply the drive signal. The drive voltage waveform usually differs for different optical transitions of the pixel. The range of drive waveforms, or drive signals, that are needed for full addressing of the display is typically stored in a look-up-table taking the present state and the subsequent state as input and specifying a suitable waveform based thereon.
In order to provide smooth transitions between pixel images, short updating times are crucial. However, drive waveforms including the above-described shaking and preset pulses of course extend the updating time. A tradeoff thus has to made between image updating time and accurate image updating.